The goal of the present project is to use neuroimaging methods, specifically positron emission tomography (PET), to clarify the neural mechanisms mediating age-related cognitive changes in healthy older adults. In each of three experiments, 40 young and 40 older adult men will receive a neurological exam, and psychometric and cognitive testing. Within each age group, 25 subjects will receive a magnetic resonance imaging (MRI) scan. PET testing will be scheduled for 12-13 subjects per age group, based on exclusionary criteria established for the psychometric testing and MRI scan. The PET assessments will use a high resolution tomograph, MRI-registered images, and arterial blood samples. Each subject's PET study will involve six 15O-water scans separated by 15-min intervals. Four of those scans will be activation scans in which subjects view a series of stimuli presented on a computer monitor and make a choice response during the PET scan. Two of the scans will be control conditions. The data of primary interest will be the pattern of change in regional cerebral blood flow (rCBF), across task conditions, as a function of age group. Both region-of-interest and exploratory (e.g., statistical parametric mapping) analyses will be conducted. Experiment 1 will examine the discrimination of visually presented words and nonwords. It is expected that an age-related decline in rCBF will be most evident, in areas of striate and extrastriate cortex, when there is a pronounced feature-extraction component to the word identification task. Experiment 2 will investigate memory encoding and retrieval of visually presented word lists. The prediction is that an age-related decline in rCBF in areas of medial temporal and frontal cortex will be evident when either encoding or retrieval demands are high. Experiment 3 will examine divided and selective attention processes in the context of a visual search task. It is predicted that the requirement to engage divided attention processes will be associated with an age-related decline in rCBF in areas of frontal cortex (e.g., anterior cingulate). The experiments proposed in the present application are designed to provide new information regarding the functional neuroanatomy of age- related change is necessary if improved methods are to be developed for identifying neuropathological conditions such as Alzheimer's disease.